summaryrefslogtreecommitdiff
path: root/block/blk-mq-sched.c
blob: deff4e826e234dea5ed72dbb041ca7d3e1768a1d (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
// SPDX-License-Identifier: GPL-2.0
/*
 * blk-mq scheduling framework
 *
 * Copyright (C) 2016 Jens Axboe
 */
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/blk-mq.h>
#include <linux/list_sort.h>

#include <trace/events/block.h>

#include "blk.h"
#include "blk-mq.h"
#include "blk-mq-debugfs.h"
#include "blk-mq-sched.h"
#include "blk-mq-tag.h"
#include "blk-wbt.h"

void blk_mq_sched_assign_ioc(struct request *rq)
{
	struct request_queue *q = rq->q;
	struct io_context *ioc;
	struct io_cq *icq;

	/*
	 * May not have an IO context if it's a passthrough request
	 */
	ioc = current->io_context;
	if (!ioc)
		return;

	spin_lock_irq(&q->queue_lock);
	icq = ioc_lookup_icq(ioc, q);
	spin_unlock_irq(&q->queue_lock);

	if (!icq) {
		icq = ioc_create_icq(ioc, q, GFP_ATOMIC);
		if (!icq)
			return;
	}
	get_io_context(icq->ioc);
	rq->elv.icq = icq;
}

/*
 * Mark a hardware queue as needing a restart. For shared queues, maintain
 * a count of how many hardware queues are marked for restart.
 */
void blk_mq_sched_mark_restart_hctx(struct blk_mq_hw_ctx *hctx)
{
	if (test_bit(BLK_MQ_S_SCHED_RESTART, &hctx->state))
		return;

	set_bit(BLK_MQ_S_SCHED_RESTART, &hctx->state);
}
EXPORT_SYMBOL_GPL(blk_mq_sched_mark_restart_hctx);

void blk_mq_sched_restart(struct blk_mq_hw_ctx *hctx)
{
	if (!test_bit(BLK_MQ_S_SCHED_RESTART, &hctx->state))
		return;
	clear_bit(BLK_MQ_S_SCHED_RESTART, &hctx->state);

	/*
	 * Order clearing SCHED_RESTART and list_empty_careful(&hctx->dispatch)
	 * in blk_mq_run_hw_queue(). Its pair is the barrier in
	 * blk_mq_dispatch_rq_list(). So dispatch code won't see SCHED_RESTART,
	 * meantime new request added to hctx->dispatch is missed to check in
	 * blk_mq_run_hw_queue().
	 */
	smp_mb();

	blk_mq_run_hw_queue(hctx, true);
}

static int sched_rq_cmp(void *priv, struct list_head *a, struct list_head *b)
{
	struct request *rqa = container_of(a, struct request, queuelist);
	struct request *rqb = container_of(b, struct request, queuelist);

	return rqa->mq_hctx > rqb->mq_hctx;
}

static bool blk_mq_dispatch_hctx_list(struct list_head *rq_list)
{
	struct blk_mq_hw_ctx *hctx =
		list_first_entry(rq_list, struct request, queuelist)->mq_hctx;
	struct request *rq;
	LIST_HEAD(hctx_list);
	unsigned int count = 0;

	list_for_each_entry(rq, rq_list, queuelist) {
		if (rq->mq_hctx != hctx) {
			list_cut_before(&hctx_list, rq_list, &rq->queuelist);
			goto dispatch;
		}
		count++;
	}
	list_splice_tail_init(rq_list, &hctx_list);

dispatch:
	return blk_mq_dispatch_rq_list(hctx, &hctx_list, count);
}

#define BLK_MQ_BUDGET_DELAY	3		/* ms units */

/*
 * Only SCSI implements .get_budget and .put_budget, and SCSI restarts
 * its queue by itself in its completion handler, so we don't need to
 * restart queue if .get_budget() returns BLK_STS_NO_RESOURCE.
 *
 * Returns -EAGAIN if hctx->dispatch was found non-empty and run_work has to
 * be run again.  This is necessary to avoid starving flushes.
 */
static int __blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx)
{
	struct request_queue *q = hctx->queue;
	struct elevator_queue *e = q->elevator;
	bool multi_hctxs = false, run_queue = false;
	bool dispatched = false, busy = false;
	unsigned int max_dispatch;
	LIST_HEAD(rq_list);
	int count = 0;

	if (hctx->dispatch_busy)
		max_dispatch = 1;
	else
		max_dispatch = hctx->queue->nr_requests;

	do {
		struct request *rq;

		if (e->type->ops.has_work && !e->type->ops.has_work(hctx))
			break;

		if (!list_empty_careful(&hctx->dispatch)) {
			busy = true;
			break;
		}

		if (!blk_mq_get_dispatch_budget(q))
			break;

		rq = e->type->ops.dispatch_request(hctx);
		if (!rq) {
			blk_mq_put_dispatch_budget(q);
			/*
			 * We're releasing without dispatching. Holding the
			 * budget could have blocked any "hctx"s with the
			 * same queue and if we didn't dispatch then there's
			 * no guarantee anyone will kick the queue.  Kick it
			 * ourselves.
			 */
			run_queue = true;
			break;
		}

		/*
		 * Now this rq owns the budget which has to be released
		 * if this rq won't be queued to driver via .queue_rq()
		 * in blk_mq_dispatch_rq_list().
		 */
		list_add_tail(&rq->queuelist, &rq_list);
		if (rq->mq_hctx != hctx)
			multi_hctxs = true;
	} while (++count < max_dispatch);

	if (!count) {
		if (run_queue)
			blk_mq_delay_run_hw_queues(q, BLK_MQ_BUDGET_DELAY);
	} else if (multi_hctxs) {
		/*
		 * Requests from different hctx may be dequeued from some
		 * schedulers, such as bfq and deadline.
		 *
		 * Sort the requests in the list according to their hctx,
		 * dispatch batching requests from same hctx at a time.
		 */
		list_sort(NULL, &rq_list, sched_rq_cmp);
		do {
			dispatched |= blk_mq_dispatch_hctx_list(&rq_list);
		} while (!list_empty(&rq_list));
	} else {
		dispatched = blk_mq_dispatch_rq_list(hctx, &rq_list, count);
	}

	if (busy)
		return -EAGAIN;
	return !!dispatched;
}

static int blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx)
{
	int ret;

	do {
		ret = __blk_mq_do_dispatch_sched(hctx);
	} while (ret == 1);

	return ret;
}

static struct blk_mq_ctx *blk_mq_next_ctx(struct blk_mq_hw_ctx *hctx,
					  struct blk_mq_ctx *ctx)
{
	unsigned short idx = ctx->index_hw[hctx->type];

	if (++idx == hctx->nr_ctx)
		idx = 0;

	return hctx->ctxs[idx];
}

/*
 * Only SCSI implements .get_budget and .put_budget, and SCSI restarts
 * its queue by itself in its completion handler, so we don't need to
 * restart queue if .get_budget() returns BLK_STS_NO_RESOURCE.
 *
 * Returns -EAGAIN if hctx->dispatch was found non-empty and run_work has to
 * be run again.  This is necessary to avoid starving flushes.
 */
static int blk_mq_do_dispatch_ctx(struct blk_mq_hw_ctx *hctx)
{
	struct request_queue *q = hctx->queue;
	LIST_HEAD(rq_list);
	struct blk_mq_ctx *ctx = READ_ONCE(hctx->dispatch_from);
	int ret = 0;
	struct request *rq;

	do {
		if (!list_empty_careful(&hctx->dispatch)) {
			ret = -EAGAIN;
			break;
		}

		if (!sbitmap_any_bit_set(&hctx->ctx_map))
			break;

		if (!blk_mq_get_dispatch_budget(q))
			break;

		rq = blk_mq_dequeue_from_ctx(hctx, ctx);
		if (!rq) {
			blk_mq_put_dispatch_budget(q);
			/*
			 * We're releasing without dispatching. Holding the
			 * budget could have blocked any "hctx"s with the
			 * same queue and if we didn't dispatch then there's
			 * no guarantee anyone will kick the queue.  Kick it
			 * ourselves.
			 */
			blk_mq_delay_run_hw_queues(q, BLK_MQ_BUDGET_DELAY);
			break;
		}

		/*
		 * Now this rq owns the budget which has to be released
		 * if this rq won't be queued to driver via .queue_rq()
		 * in blk_mq_dispatch_rq_list().
		 */
		list_add(&rq->queuelist, &rq_list);

		/* round robin for fair dispatch */
		ctx = blk_mq_next_ctx(hctx, rq->mq_ctx);

	} while (blk_mq_dispatch_rq_list(rq->mq_hctx, &rq_list, 1));

	WRITE_ONCE(hctx->dispatch_from, ctx);
	return ret;
}

static int __blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx)
{
	struct request_queue *q = hctx->queue;
	struct elevator_queue *e = q->elevator;
	const bool has_sched_dispatch = e && e->type->ops.dispatch_request;
	int ret = 0;
	LIST_HEAD(rq_list);

	/*
	 * If we have previous entries on our dispatch list, grab them first for
	 * more fair dispatch.
	 */
	if (!list_empty_careful(&hctx->dispatch)) {
		spin_lock(&hctx->lock);
		if (!list_empty(&hctx->dispatch))
			list_splice_init(&hctx->dispatch, &rq_list);
		spin_unlock(&hctx->lock);
	}

	/*
	 * Only ask the scheduler for requests, if we didn't have residual
	 * requests from the dispatch list. This is to avoid the case where
	 * we only ever dispatch a fraction of the requests available because
	 * of low device queue depth. Once we pull requests out of the IO
	 * scheduler, we can no longer merge or sort them. So it's best to
	 * leave them there for as long as we can. Mark the hw queue as
	 * needing a restart in that case.
	 *
	 * We want to dispatch from the scheduler if there was nothing
	 * on the dispatch list or we were able to dispatch from the
	 * dispatch list.
	 */
	if (!list_empty(&rq_list)) {
		blk_mq_sched_mark_restart_hctx(hctx);
		if (blk_mq_dispatch_rq_list(hctx, &rq_list, 0)) {
			if (has_sched_dispatch)
				ret = blk_mq_do_dispatch_sched(hctx);
			else
				ret = blk_mq_do_dispatch_ctx(hctx);
		}
	} else if (has_sched_dispatch) {
		ret = blk_mq_do_dispatch_sched(hctx);
	} else if (hctx->dispatch_busy) {
		/* dequeue request one by one from sw queue if queue is busy */
		ret = blk_mq_do_dispatch_ctx(hctx);
	} else {
		blk_mq_flush_busy_ctxs(hctx, &rq_list);
		blk_mq_dispatch_rq_list(hctx, &rq_list, 0);
	}

	return ret;
}

void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx)
{
	struct request_queue *q = hctx->queue;

	/* RCU or SRCU read lock is needed before checking quiesced flag */
	if (unlikely(blk_mq_hctx_stopped(hctx) || blk_queue_quiesced(q)))
		return;

	hctx->run++;

	/*
	 * A return of -EAGAIN is an indication that hctx->dispatch is not
	 * empty and we must run again in order to avoid starving flushes.
	 */
	if (__blk_mq_sched_dispatch_requests(hctx) == -EAGAIN) {
		if (__blk_mq_sched_dispatch_requests(hctx) == -EAGAIN)
			blk_mq_run_hw_queue(hctx, true);
	}
}

bool __blk_mq_sched_bio_merge(struct request_queue *q, struct bio *bio,
		unsigned int nr_segs)
{
	struct elevator_queue *e = q->elevator;
	struct blk_mq_ctx *ctx = blk_mq_get_ctx(q);
	struct blk_mq_hw_ctx *hctx = blk_mq_map_queue(q, bio->bi_opf, ctx);
	bool ret = false;
	enum hctx_type type;

	if (e && e->type->ops.bio_merge)
		return e->type->ops.bio_merge(hctx, bio, nr_segs);

	type = hctx->type;
	if (!(hctx->flags & BLK_MQ_F_SHOULD_MERGE) ||
	    list_empty_careful(&ctx->rq_lists[type]))
		return false;

	/* default per sw-queue merge */
	spin_lock(&ctx->lock);
	/*
	 * Reverse check our software queue for entries that we could
	 * potentially merge with. Currently includes a hand-wavy stop
	 * count of 8, to not spend too much time checking for merges.
	 */
	if (blk_bio_list_merge(q, &ctx->rq_lists[type], bio, nr_segs)) {
		ctx->rq_merged++;
		ret = true;
	}

	spin_unlock(&ctx->lock);

	return ret;
}

bool blk_mq_sched_try_insert_merge(struct request_queue *q, struct request *rq)
{
	return rq_mergeable(rq) && elv_attempt_insert_merge(q, rq);
}
EXPORT_SYMBOL_GPL(blk_mq_sched_try_insert_merge);

void blk_mq_sched_request_inserted(struct request *rq)
{
	trace_block_rq_insert(rq);
}
EXPORT_SYMBOL_GPL(blk_mq_sched_request_inserted);

static bool blk_mq_sched_bypass_insert(struct blk_mq_hw_ctx *hctx,
				       bool has_sched,
				       struct request *rq)
{
	/*
	 * dispatch flush and passthrough rq directly
	 *
	 * passthrough request has to be added to hctx->dispatch directly.
	 * For some reason, device may be in one situation which can't
	 * handle FS request, so STS_RESOURCE is always returned and the
	 * FS request will be added to hctx->dispatch. However passthrough
	 * request may be required at that time for fixing the problem. If
	 * passthrough request is added to scheduler queue, there isn't any
	 * chance to dispatch it given we prioritize requests in hctx->dispatch.
	 */
	if ((rq->rq_flags & RQF_FLUSH_SEQ) || blk_rq_is_passthrough(rq))
		return true;

	if (has_sched)
		rq->rq_flags |= RQF_SORTED;

	return false;
}

void blk_mq_sched_insert_request(struct request *rq, bool at_head,
				 bool run_queue, bool async)
{
	struct request_queue *q = rq->q;
	struct elevator_queue *e = q->elevator;
	struct blk_mq_ctx *ctx = rq->mq_ctx;
	struct blk_mq_hw_ctx *hctx = rq->mq_hctx;

	WARN_ON(e && (rq->tag != BLK_MQ_NO_TAG));

	if (blk_mq_sched_bypass_insert(hctx, !!e, rq)) {
		/*
		 * Firstly normal IO request is inserted to scheduler queue or
		 * sw queue, meantime we add flush request to dispatch queue(
		 * hctx->dispatch) directly and there is at most one in-flight
		 * flush request for each hw queue, so it doesn't matter to add
		 * flush request to tail or front of the dispatch queue.
		 *
		 * Secondly in case of NCQ, flush request belongs to non-NCQ
		 * command, and queueing it will fail when there is any
		 * in-flight normal IO request(NCQ command). When adding flush
		 * rq to the front of hctx->dispatch, it is easier to introduce
		 * extra time to flush rq's latency because of S_SCHED_RESTART
		 * compared with adding to the tail of dispatch queue, then
		 * chance of flush merge is increased, and less flush requests
		 * will be issued to controller. It is observed that ~10% time
		 * is saved in blktests block/004 on disk attached to AHCI/NCQ
		 * drive when adding flush rq to the front of hctx->dispatch.
		 *
		 * Simply queue flush rq to the front of hctx->dispatch so that
		 * intensive flush workloads can benefit in case of NCQ HW.
		 */
		at_head = (rq->rq_flags & RQF_FLUSH_SEQ) ? true : at_head;
		blk_mq_request_bypass_insert(rq, at_head, false);
		goto run;
	}

	if (e && e->type->ops.insert_requests) {
		LIST_HEAD(list);

		list_add(&rq->queuelist, &list);
		e->type->ops.insert_requests(hctx, &list, at_head);
	} else {
		spin_lock(&ctx->lock);
		__blk_mq_insert_request(hctx, rq, at_head);
		spin_unlock(&ctx->lock);
	}

run:
	if (run_queue)
		blk_mq_run_hw_queue(hctx, async);
}

void blk_mq_sched_insert_requests(struct blk_mq_hw_ctx *hctx,
				  struct blk_mq_ctx *ctx,
				  struct list_head *list, bool run_queue_async)
{
	struct elevator_queue *e;
	struct request_queue *q = hctx->queue;

	/*
	 * blk_mq_sched_insert_requests() is called from flush plug
	 * context only, and hold one usage counter to prevent queue
	 * from being released.
	 */
	percpu_ref_get(&q->q_usage_counter);

	e = hctx->queue->elevator;
	if (e && e->type->ops.insert_requests)
		e->type->ops.insert_requests(hctx, list, false);
	else {
		/*
		 * try to issue requests directly if the hw queue isn't
		 * busy in case of 'none' scheduler, and this way may save
		 * us one extra enqueue & dequeue to sw queue.
		 */
		if (!hctx->dispatch_busy && !e && !run_queue_async) {
			blk_mq_try_issue_list_directly(hctx, list);
			if (list_empty(list))
				goto out;
		}
		blk_mq_insert_requests(hctx, ctx, list);
	}

	blk_mq_run_hw_queue(hctx, run_queue_async);
 out:
	percpu_ref_put(&q->q_usage_counter);
}

static void blk_mq_sched_free_tags(struct blk_mq_tag_set *set,
				   struct blk_mq_hw_ctx *hctx,
				   unsigned int hctx_idx)
{
	unsigned int flags = set->flags & ~BLK_MQ_F_TAG_HCTX_SHARED;

	if (hctx->sched_tags) {
		blk_mq_free_rqs(set, hctx->sched_tags, hctx_idx);
		blk_mq_free_rq_map(hctx->sched_tags, flags);
		hctx->sched_tags = NULL;
	}
}

static int blk_mq_sched_alloc_tags(struct request_queue *q,
				   struct blk_mq_hw_ctx *hctx,
				   unsigned int hctx_idx)
{
	struct blk_mq_tag_set *set = q->tag_set;
	/* Clear HCTX_SHARED so tags are init'ed */
	unsigned int flags = set->flags & ~BLK_MQ_F_TAG_HCTX_SHARED;
	int ret;

	hctx->sched_tags = blk_mq_alloc_rq_map(set, hctx_idx, q->nr_requests,
					       set->reserved_tags, flags);
	if (!hctx->sched_tags)
		return -ENOMEM;

	ret = blk_mq_alloc_rqs(set, hctx->sched_tags, hctx_idx, q->nr_requests);
	if (ret)
		blk_mq_sched_free_tags(set, hctx, hctx_idx);

	return ret;
}

/* called in queue's release handler, tagset has gone away */
static void blk_mq_sched_tags_teardown(struct request_queue *q)
{
	struct blk_mq_hw_ctx *hctx;
	int i;

	queue_for_each_hw_ctx(q, hctx, i) {
		/* Clear HCTX_SHARED so tags are freed */
		unsigned int flags = hctx->flags & ~BLK_MQ_F_TAG_HCTX_SHARED;

		if (hctx->sched_tags) {
			blk_mq_free_rq_map(hctx->sched_tags, flags);
			hctx->sched_tags = NULL;
		}
	}
}

int blk_mq_init_sched(struct request_queue *q, struct elevator_type *e)
{
	struct blk_mq_hw_ctx *hctx;
	struct elevator_queue *eq;
	unsigned int i;
	int ret;

	if (!e) {
		q->elevator = NULL;
		q->nr_requests = q->tag_set->queue_depth;
		return 0;
	}

	/*
	 * Default to double of smaller one between hw queue_depth and 128,
	 * since we don't split into sync/async like the old code did.
	 * Additionally, this is a per-hw queue depth.
	 */
	q->nr_requests = 2 * min_t(unsigned int, q->tag_set->queue_depth,
				   BLKDEV_MAX_RQ);

	queue_for_each_hw_ctx(q, hctx, i) {
		ret = blk_mq_sched_alloc_tags(q, hctx, i);
		if (ret)
			goto err;
	}

	ret = e->ops.init_sched(q, e);
	if (ret)
		goto err;

	blk_mq_debugfs_register_sched(q);

	queue_for_each_hw_ctx(q, hctx, i) {
		if (e->ops.init_hctx) {
			ret = e->ops.init_hctx(hctx, i);
			if (ret) {
				eq = q->elevator;
				blk_mq_sched_free_requests(q);
				blk_mq_exit_sched(q, eq);
				kobject_put(&eq->kobj);
				return ret;
			}
		}
		blk_mq_debugfs_register_sched_hctx(q, hctx);
	}

	return 0;

err:
	blk_mq_sched_free_requests(q);
	blk_mq_sched_tags_teardown(q);
	q->elevator = NULL;
	return ret;
}

/*
 * called in either blk_queue_cleanup or elevator_switch, tagset
 * is required for freeing requests
 */
void blk_mq_sched_free_requests(struct request_queue *q)
{
	struct blk_mq_hw_ctx *hctx;
	int i;

	queue_for_each_hw_ctx(q, hctx, i) {
		if (hctx->sched_tags)
			blk_mq_free_rqs(q->tag_set, hctx->sched_tags, i);
	}
}

void blk_mq_exit_sched(struct request_queue *q, struct elevator_queue *e)
{
	struct blk_mq_hw_ctx *hctx;
	unsigned int i;

	queue_for_each_hw_ctx(q, hctx, i) {
		blk_mq_debugfs_unregister_sched_hctx(hctx);
		if (e->type->ops.exit_hctx && hctx->sched_data) {
			e->type->ops.exit_hctx(hctx, i);
			hctx->sched_data = NULL;
		}
	}
	blk_mq_debugfs_unregister_sched(q);
	if (e->type->ops.exit_sched)
		e->type->ops.exit_sched(e);
	blk_mq_sched_tags_teardown(q);
	q->elevator = NULL;
}